Recently, as a new communications means, UWB has been attracting attention. UWB realizes the transfer of large volumes of data over short distances of approximately 10 m by using a wide frequency band (e.g., a frequency band ranging from 3.1 to 10.6 GHz) and there are plans for its use according to the regulations of the FCC (Federal Communication Commission) in the U.S. In this way, UWB is characterized by using a substantially wide frequency band.
Research on bandpass filters having a substantially wide passband and can be used for such UWB has been actively carried out in recent years, and in one study, for example, it was reported that a characteristic feature in which a substantially wide passband with a passband width exceeding 100% of the fractional bandwidth (bandwidth/center frequency) was obtained by using a bandpass filter to which the principles of a directional coupler were applied (e.g., refer to “An ultra-wideband bandpass filter using a microstrip-CPW broadside coupled structure”, Proceedings of the March 2005 IEICE General Conference, C-2-114, p. 147).
Meanwhile, as a commonly used conventional bandpass filter, one in which a plurality of ¼ wavelength stripline resonators is installed in parallel and configured to couple with each other is known (e.g., refer to Unexamined Patent Publication No. 2004-180032).
However, the bandpass filters proposed in “An ultra-wideband bandpass filter using a microstrip-CPW broadside coupled structure” (Proceeding of the March 2005 IEICE General Conference, C-2-114 p. 147) and Unexamined Patent Publication No. 2004-180032 each had problems and were not appropriate for UWB.
For example, the bandpass filter proposed in “An ultra-wideband bandpass filter using a microstrip-CPW broadside coupled structure” (Proceeding of the March 2005 IEICE General Conference, C-2-114 p. 147) had a problem in that the passband width was too wide. In other words, the UWB basically uses a frequency band ranging from 3.1 GHz to 10.6 GHz, whereas the Radiocommunications Sector of the International Telecommunication Union proposes a plan to demultiplex into Low Band using a frequency band ranging from approximately 3.1 to 4.7 GHz and High Band using a frequency band ranging from approximately 6 GHz to 10.6 GHz, thus avoiding the use of 5.3 GHz at IEEE802.11.a. Accordingly, because a filter for Low Band that allows Low Band to pass and a filter for High Band that allows High Band to pass each required both a passband width ranging from approximately 40% to 50% of the fractional bandwidth and attenuation at 5.3 GHz, the bandpass filter proposed in “A ultra-wideband bandpass filter using a microstrip-CPW broadside coupled structure” (Proceeding of the March 2005 IEICE General Conference C-2-114 p. 147) having a characteristic feature with a passband width greater than 100% of the fractional bandwidth cannot be used due to its wide passband width.
Additionally, the passband width of the bandpass filter using a conventional ¼ wavelength resonator is too narrow, and even the passband width of the bandpass filter described in Unexamined Patent Publication No. 2004-180032, which attempted to provide a wider bandwidth, was less than 10% of the fractional bandwidth. Accordingly, it cannot be used as a bandpass filter for UWB, which requires a wide passband width corresponding to 40% to 50% of the fractional bandwidth.
Furthermore, if both Low Band and High Band are used, because a circuit processing Low Band signals and a circuit processing High Band signals are different in the RF IC that processes radio-frequency signals, the antenna side may have two terminals, thus increasing the need for a diplexer that connects the terminal on the Low Band side and the terminal on the High Band side to the antenna. In addition, such a diplexer requires that isolation between the terminal on the Low Band side and the terminal on the High Band side is sufficiently secured.